1. Field of the Invention
The present invention relates to a double telecentric objective lens.
2. Description of the Related Art
With recent developments in image processing technology, various image processing and measuring apparatuses have been commercialized. Objects to be measured are diversified and complicated, and there is a need to measure, by image processing and measuring apparatuses, relatively large and thick machine parts, cutting tools, electronic parts, and the like that have hitherto been measured by measuring projectors.
For this reason, the emergence of a telecentric objective lens having a low magnification of 1xc3x97 or less is required, in which the field of view is wide, the depth of field is large enough to measure thick machine parts having a stepped portion, and the like, and the telecentricity (the degree of parallelizm between the principal ray of an off-axis beam and the optical axis) is properly corrected.
Preferable as a telecentric objective lens used for measurement is a so-called double telecentric objective optical system in which lenses are divided into two groups, a front group and a rear group, the back focus of the front group and the front focus point of the rear group coincide with each other, and a diaphragm is placed at the position where the focus points coincide. This is because the image magnification of the double telecentric optical system is basically determined only by the focal lengths of the front group and the rear group, regardless of the position of an object.
In view of such circumstances, it is an object of the present invention to provide a double telecentric objective lens having a magnification of approximately 0.2xc3x97 in which aberrations and the telecentricity of the principal ray of an off-axis beam are properly corrected, and which is suitable for use in an image processing and measuring apparatus.
In order to achieve the above object, according to one aspect, the present invention provides a double telecentric objective lens having a double telecentric optical system. The double telecentric optical system includes a front group having a positive refracting power as a whole, and a rear group having a positive refracting power as a whole. The rear focus of the front group and the front focus of the rear group coincide with each other, and a diaphragm is placed at the position where the focuses coincide. The front group includes a first unit formed of a cemented lens composed of a convex lens and a concave lens, and having a positive refracting power as a whole, and a second unit including a convex lens or a cemented lens composed of a convex lens and a convex lens, and a concave lens, arranged in that order from the object side, and having a positive refracting power as a whole. The rear group includes a third unit including a concave lens and a cemented lens composed of a convex lens and a concave lens, and having a positive refracting power as a whole, and a fourth unit formed of a cemented lens composed of a convex lens and a concave lens, and having a positive refracting power as a whole.
The double telecentric objective lens satisfies the following conditions:
n1nxe2x88x92n1p greater than 0.1xe2x80x83xe2x80x83(1)
xcexd1pxe2x88x92xcexd1n greater than 25xe2x80x83xe2x80x83(2)
0.3fF less than |r2n| less than 0.5fFxe2x80x83xe2x80x83(3)
1.4 less than (r2p/r2n) less than 2.7xe2x80x83xe2x80x83(4)
n3n less than n3pxe2x80x83xe2x80x83(5)
where n1p and xcexd1p respectively represent the refractive index and the Abbe""s number of the convex lens of the first unit, n1n and xcexd1n respectively represent the refractive index and the Abbe""s number of the concave lens of the first unit, r2p represents the radius of curvature of a surface of the convex lens or the cemented lens composed of the convex lens and the concave lens in the second unit that is furthermost from an object, r2n represents the radius of curvature of an object-side surface of the concave lens in the second unit, fF represents the focal length of the entire front group, n3n represents the average refractive index of the concave lenses in the third unit, n3p represents the refractive index of the convex lens in the third unit, and the refractive indices and the focal length are values for the d-line (587.56 nm).
Conditional expression (1) specifies the refractive indices of the lenses included in the first unit.
If conditional expression (1) is not satisfied, the difference in refractive index between the convex lens and the concave lens decreases. Therefore, when spherical aberration and the telecentricity of the principal ray are corrected, the radius of curvature of the bonding surface decreases, and a high-order aberration, such as spherical aberration, occurs. Moreover, since the principal ray emitted from the vicinity of the object and in parallel with the optical axis greatly deviates from the center of the telecentric diaphragm, the principal ray of a light beam passing through the center of the telecentric diaphragm forms a large angle with respect to the optical axis.
When an object having a stepped portion is measured with such an objective lens, a serious measurement error occurs around the object.
Conditional expression (2) specifies the Abbe""s numbers of the lenses used in the first unit.
If chromatic aberration is corrected using glass materials that do not satisfy conditional expression (2), the refractive powers of both the convex lens and the concave lens must be increased, and this results in a high-order aberration such as spherical aberration or comatic aberration. The telecentricity of an off-axis beam also deteriorates. The high-order aberration cannot be completely corrected by other lens units.
In other words, conditional expressions (1) and (2) are determined so as to minimize spherical aberration, comatic aberration, and chromatic aberration, and the deterioration of telecentricity of an off-axis principal ray.
Conditional expression (3) specifies the radius of curvature of an object-side surface of the concave lens in the second unit.
When the radius of curvature exceeds the upper limit in conditional expression (3), spherical aberration or the like that occurs at another position is not sufficiently corrected, and correction of aberration in the entire front group is insufficient. The telecentricity of the principal ray is not also completely corrected.
When the radius of curvature falls below the lower limit in conditional expression (3), an excessive positive aberration occurs at this surface. In this case, the balance cannot be achieved even by producing negative aberrations at another surface, or a large high-order aberration occurs.
Conditional expression (4) specifies the radius of curvature r2p of a surface of the convex lens or the cemented lens composed of the convex lens and the concave lens that is furthermost from the object in the second unit, and the radius of curvature r2n of an object-side surface of the concave lens in the second unit.
This condition is necessary to achieve a balance between negative aberrations caused at the surface having the radius of curvature r2p and positive aberrations caused at the surface having the radius of curvature r2n, to correct aberrations in the entire front group including residual aberration caused in the first unit, and to maintain high telecentricity of the principal ray emitted from the object.
Positive aberrations at the surface with r2n increase above the upper limit in conditional expression (4), and negative aberrations at the surface with r2p increase below the lower limit. When any of the aberrations is compensated for in another unit, the degree of compensation increases. Consequently, high-order aberration cannot be prevented, and the telecentricity of the principal ray of an off-axis beam deteriorates.
That is, the conditions in conditional expressions (3) and (4) are necessary to maintain the balance of the aberrations in the entire front group, and to properly correct the telecentricity of the principal ray of the off-axis beam.
Conditional expression (5) specifies the refractive indices of the convex lens and the concave lenses in the third unit.
In order to minimize aberrations in the third unit including high-order aberrations, such as spherical aberration and comatic aberration, the convex lens is made of a glass material having a high refractive index. Further, in order to limit the Petzval sum so as to properly correct astigmatism, the convex lens is made of a glass material having a high refractive index, and the concave lenses are made of a glass material having a low refractive index.
When the convex lens and the concave lenses are made of glass materials that do not satisfy conditional expression (5), the Petzval sum increases, and therefore, astigmatism increases.
This condition is necessary, in particular, to properly correct astigmatism in the entire optical system including the front group and the rear group.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.